At cell division, or mitosis, the nucleus of the cell divides. Each chromosome within the nucleus is first duplicated, and one copy passes to each daughter nucleus, and hence to each daughter cell. 

In meisos, the process whereby sperm and egg are formed, only half of the chromosomes are passed along. Only when egg and sperm are combined is the full genetic inheritance restored. Each regular cell in the body, called a somatic cell, has its pair of chromosomes, one from the father, and one from the mother. When egg or sperm cells are formed, either the maternal or paternal chromosome of each pair is chosen at random and passed along. This is the mechanism of genetics. (see genotype / phenotype. )